1. Field of the Invention
The present invention relates to photopolymerizable compositions and, particularly, to photopolymerizable compositions containing a photopolymerization initiator having a novel composition, which is sensitive to, for example, argon laser beams.
2. Description of the Prior Art
It has been well known that images can be formed by a process which comprises preparing a photopolymerizable composition comprising a compound having at least one ethylenically unsaturated double bond capable of undergoing addition polymerization (hereinafter "ethylenic compound"), a photopolymerization initiator and, if necessary, a binder polymer having film forming ability, a thermal polymerization inhibitor or a plasticizer, etc.; preparing a solution of the photopolymerizable composition, applying the solution to a support to produce a photosensitive material having a photopolymerizable composition layer (photosensitive layer), imagewise exposing the sensitive layer to light according to the desired original to harden the exposed portions by polymerization, and processing with an organic solvent capable of dissolving only the unhardened portions to remove the unhardened portions by dissolution, by which images composed of the hardened portions are formed.
It has also been known that images can be formed by a process which comprises forming a laminate by pressing the photosensitive material described above to an image support (one of the support of the photosensitive material and the image support is transparent), exposing it imagewise to light through the transparent support side to polymerize the exposed portions of the sensitive layer, by which the adhesion to the support in the photosensitive material and the adhesion to the image support change, and the adhesive strength in the exposed portions becomes different from that in the unexposed portions; and separating both supports by stripping in the state that the unexposed portions of the sensitive layer adhere to one support and the exposed portions of the sensitive layer adhere to the other support, by which images composed of the sensitive layer are formed.
In such processes, benzyl, benzoin, anthraquinone and Michler's ketone, etc., have been used as photopolymerization initiators. However, photopolymerizable compositions containing these photopolymerization initiators are hardly sensitive to ultraviolet radiation having long wavelengths or visible radiation, for example, they are hardly sensitive to argon laser beams, and do not have a polymerization initiating ability, though they are sensitive to ultraviolet radiation having comparatively short wavelengths. It is very desirable to use visible radiation as light sources for exposure in image formation or light sources capable of scanning, for example, laser beams, and for this reason prior art photopolymerization initiators have not been satisfactory.